Stromatolite Formation Research Articles

Selective Dolomitization of Cambrian Microbial Carbonate Deposits: A Key to Mechanisms and Environments of Origin

The Upper Cambrian Maynardville Limestone (Conasauga Group) and the lower Copper Ridge Dolomite (Knox Group) of the southern Appalachians contain a variety of microbial carbonate deposits, such as stratiform (cryptalgal laminates), laterally linked hemispheroidal (LLH), vertically stacked hemispheroidal (SH), columnar, and digitate stromatolites, as well as thrombolites. Digitate stromatolites and thrombolites, in most cases, do not exhibit evidence of significant dolomitization, even though they are commonly embedded within completely dolomitized deposits. These microbial deposits formed primarily by calcification of cyanobacteria in lower intertidal and upper subtidal environments, which were not primary sites for dolomitization. Early diagenetic calcification of cyanobacteria additionally reduced the susceptibility of these deposits to dolomitization. Extensively dolomitized microbial deposits (LLH, SH, and columnar stromatolites, and most of stratiform stromatolite laminae) formed primarily by the trapping of sediment in supratidal and intertidal environments on arid to semi-arid tidal flats. Pervasive syndepositional calcification of cyanobacteria may have been precluded by conditions of periodic emergence, hypersalinity, and the presence of finegrained sediment serving as competing sites for carbonate mineral nucleation. Extensive dolomitization altered these peritidal carbonate sediments early in their diagenetic history (penecontemporaneous or syngenetic dolomitization). The presence of both calcitic and dolomitic laminae is indicative of a combination of trapping and calcification for the formation of stratiform stromatolites. The formation of Upper Cambrian microbial deposits was primarily controlled by the conditions within the environments of deposition, but was also biotically influenced to some degree.

Growth History of Stromatolites in a Holocene Fringing Reef, Stocking Island, Bahamas

ABSTRACT A stromatolite and algal ridge reef complex 2.1 m thick fringes the east coast of Stocking Island, Exuma Cays, Bahamas. This reef was established on a Pleistocene calcarenite terrace about 4500 yr B.P. Stromatolites, which occur in back-reef and reef-flat zones, are up to 1 m thick and were constructed by cyanobacterial-dominated communities. Study of the growth history of these stromatolites, ranging in scope from facies analyses to details of microfabric construction, presents new perspectives on stromatolite formation. Lithologies identified in eight cores from across the Stocking Island reef complex, together with plots of 13 radiocarbon dates in relation to a Bahamian sea-level curve, indicate that this reef began as an intertidal vermetid gastropod buildup. Subsequent flooding of the Pleistocene terrace allowed the branching coralline alga Neogoniolithon strictum to overgrow the vermetids and eventually form an emergent algal ridge about 1500 years ago. Shifting sands accumulated in the lee of this ridge and excluded most benthic communities and herbivores, thereby promoting growth of cyanobacterial mats that formed stromatolite buildups. With a decrease in wave energy over the last 500 years, possibly due to the growth of offshore patch reefs, the urchin Echinometra lucunter coloni ed the algal ridge. Resultant bioerosion by this urchin destroyed the emergent part of the ridge and is now undercutting the seaward edge of the stromatolite buildups. Lamination in the Stocking Island stromatolites results from early lithificafion processes in cyanobacterial mats, possibly in response to biogeochemical changes in the mats during hiatuses in sediment accretion. These processes, which create partially indurated laminae with a distinct microstructure, involve precipitation of thin micrite crusts, intense microboring along a surface below this crust, micritization of sediment grains, and precipitation of point-contact cement between micritized grains. Introduction of turf algae to the cyanobacterial mat community disrupts formation of the lithified laminae, thereby inhibiting stromatolite development.

走査型X線分析顕微鏡でみるバイオマット

Understanding the coevolution of life and the earth's environments is one of the important subjects of the Decoding the Earth Evolution Program, a grant-in-aid scientific research for the priority areas, Ministry of Education, Japan. A large amount of stromatolites and banded iron formations has been collected for this purpose. Although many of them have been regarded as remnants of microbial mats, quite few direct evidence for microbial communities, their biogeochemical roles, or ecological settings has been preserved. Here we argue that recent microbial mats and stromatolitic structures observed in many active hot springs may provide important clues for modeling stromatolite formation processes as well as Precambrian biosphere. We present some results of chemical mapping of microbial mat sampled from hot springs in central Japan. The scanning X-ray analytical microscope we introduced here provides new and important information on the morphological and chemical structures of microbial mats.

Synsedimentary Cements as Paleoproterozoic Platform Building Blocks, Pethei Group, Northwestern Canada

Peritidal and subtidal carbonates of the Pethei Group are a complex arrangement of fibrous and micritic synsedimentary precipitates, carbonate grains, and detrital micrite. Synsedimentary precipitates, present largely as stromatolitic laminae, constitute the bulk of the Pethei platform, providing about 75% of the total carbonate. They occur as stromatolitic precipitates (precipitates closely associated with stromatolitic laminae) which constitute about 60% of the rock and encrusting precipitates (precipitates outside stromatolites and between grains, i.e. cement) which constitute about 15% of the rock. The most abundant precipitates are fibrous, in the form of stromatolitic microdigitate stalks (tufa) and masses of radial fans, as well as encrusting pore-filling fans, isopachous rinds, and laminar crusts. Micrite precipitates occur as stromatolitic clotted micrite and encrusting isopachous rinds. The ratio of fibrous to micritic precipitates is 2:1. Stromatolitic precipitates are arranged in three distinct microfabrics: (1) ubiquitous, spar-micrite couplets of fibrous and clotted micrite laminae; (2) deep-water vertical fibrous masses that precipitated over micritic precipitate cores; and (3) supratidal microdigitate crusts of branching fibrous stalks. Micritic cements are interpreted to be of biogenic origin, having precipitated somehow in association with microorganisms. Fibrous precipitates are interpreted to be largely abiotic. Since stromatolitic and encrusting precipitates are petrographicaily similar, they probably share similar origins. There is little evidence of sediment trapping and binding as a mechanism for stromatolite formation. The abundance of synsedimentary precipitates varies with platform geometry: subtidal ramps > subtidal rimmed shelves > subtidal open shelves > peritidal rimmed shelves. While the basic building blocks of the Pethei platform, synsedimentary precipitates, carbonate grains, and detrital micrite, are the same as those in Phanerozoic platforms, their distribution and relative proportions are different. These differences reflect a Proterozoic carbonate factory which extended across the entire platform and which produced carbonate largely by in situ precipitation.

Mud mounds: A polygenetic spectrum of fine-grained carbonate buildups

This research report contains nine case studies (part II to X) dealing with Palaeozoic and Mesozoic mud mounds, microbial reefs, and modern zones of active micrite production, and two parts (I and XI) summarizing the major questions and results. The formation of different types ofin situ formed micrites (automicrites) in close association with siliceous sponges is documented in Devonian, Carboniferous, Triassic, Jurassic and Cretaceous mounds and suggests a common origin with a modern facies found within reef caves. Processes involved in the formation of autochthonous micrites comprise: (i) calcifying mucus enriched in Asp and Glu, this type presumably is linked to the formation of stromatolites, thrombolites and massive fabrics; (ii) protein-rich substances within confined spaces (e.g. microcavities) result in peloidal pockets, peloidal coatings and peloidal stromatolites, and (iii) decay of sponge soft tissues, presumably enriched with symbiotic bacteria, lead to the micropeloidal preservation of parts of former sponge bodies. As a consequence, there is strong evidence that the primary production of micrite in place represents the initial cause for buildup development. The mode of precipitation corresponds to biologically-induced, matrix-mediated mineralization which results in high-Mg-calcites, isotopically balanced with inorganic cements or equilibrium skeletal carbonates, respectively. If distinct automicritic fabrics are absent, the source or origin of micrite remains questionable. However, the co-occurring identifiable components are inadequate, by quantity and physiology, to explain the enhanced accumulation of fine-grained calcium carbonate. The stromatolite reefs from the Permian Zechstein Basin are regarded as reminiscent of ancestral (Precambrian) reef facies, considered the precursor of automicrite/sponge buildups. Automicrite/sponge buildups represent the basic Phanerozoic reef type. Analogous facies are still present within modern cryptic reef habitats, where the biocalcifying carbonate factory is restricted in space.

Microfossils from the Neoarchean Campbell Group, Griqualand West Sequence of the Transvaal Supergroup, and their paleoenvironmental and evolutionary implications

The oldest filament- and colonial coccoid-containing microbial fossil assemblage now known is described here from drill core samples of stromatolitic cherty limestones of the Neoarchean, ∼ 2600-Ma-old Campbell Group (Ghaap Plateau Dolomite, Lime Acres Member) obtained at Lime Acres, northern Cape Province, South Africa. The assemblage is biologically diverse, including entophysalidacean ( Eoentophysalis sp.), probable chroococcacean (unnamed colonial coccoids), and oscillatoriacean cyano-bacteria ( Eomycetopsis cf. filiformis, and Siphonophycus transvaalensis), as well as filamentous fossil bacteria ( Archaeotrichion sp.); filamentous possible microfossils (unnamed hematitic filaments) also occur. The Campbell Group microorganisms contributed to the formation of stratiform and domical to columnar stromatolitic reefs in shallow subtidal to intertidal environments of the Transvaal intracratonic sea. Although only moderately to poorly preserved, they provide new evidence regarding the paleoenvironmental setting of the Campbell Group sediments, extend the known time-range of entophysalidacean cyanobacteria by more than 400 million years, substantiate the antiquity and role in stromatolite formation of Archean oscillatoriacean cyanobacteria, and document the exceedingly slow (hypobradytelic) evolutionary rate characteristic of this early evolving prokaryotic lineage.

Kopara in Polynesian atolls: early stages of formation of calcareous stromatolites

Widespread modern calcareous stromatolites known as kopara in the vernacular language on the Tuamotu Archipelago have been found accreting on the bottom of shallow (generally < 1 m depth) lakes located on the rims of these French Polynesian atolls. They are flat, gelatinous sediments, several tens of centimetres thick, with lamination comprised of vertically alternating red organic-rich, and white carbonate-rich, millimetre-thick laminae. They originate from benthic microbial communities mainly composed of cyanobacteria, generally dominated by the genus Phormidium. The living cyanobacteria occupy only the top 1–3 cm of the deposit, the red organic matter below being made of remnants of the dead microbes, essentially of polysaccharide microfibrils inherited from the sheaths and arranged into a three-dimensional network enclosing pores ranging from a few tens of ångstroms to a few micrometres wide. The carbonates are predominantly high-Mg calcite (9–19 mole% MgCO 3) which precipitated as micron-size bunches, within the pores, on walls of the organic network. It is sometimes accompanied by aragonite (less than 22% of the total mineral fraction when present), and lesser high-Mg calcite, allochthonous bioclasts, which occasionally form up to few centimetres-thick detrital intercalations. The pore waters are variable mixtures of freshwater (rain or ground water) and sea water from the ocean or lagoon which are feeding waters of the lakes. Their chemistry deviate from a straight mixture gradient with respect to Ca 2+, Mg 2+, and alkalinity under the influence of local calcification and bacterial processes (e.g. ammonia production, sulphate reduction) within the sediments. It is hypothesized that the calcification is initiated at carboxylic sites on the walls of the polysaccharide network, the pores acting as confining organic compartments with increased internal supersaturation. Lamination is suggested to be due to the combined effects of the stratification of the microbial activity of the deposits, and alternations of fresh and saline periods within the lakes. Lacustrine environments on oceanic atolls are found to be important habitats of modern microbialite formation.

Siliciclastic Stromatolites and Thrombolites, Late Miocene, S.E. Spain

ABSTRACT Large coarse-grained siliciclastic stromatolite and thrombolite domes, forming biostromes and bioherms, occur in a marine fan-delta environment of Messinian age in the Sorbas Basin, Spain. Individual domes are up to 3 m in height and have estimated synoptic relief up to 1.5 m. They occur on a shelf-to-basin transition in the Terminal Complex, a mixed siliciclastic-carbonate unit which grades from coarse conglomeratic fan-delta and oolite deposits on the shelf, to basinal sands and silts of the Sorbas Member. Paleoslopes up to 35 degrees, extending downward for several hundred of meters to original water-depths of 70 m or more, mark the transition to the basin. Normal marine conditions are indicated by coralline algae and Porites within the domes. Thrombolites are restricted to he shelf and upper slope and are closely associated with stromatolites. The domes of the mid-slope and deeper environments are exclusively stromatolitic. Medium to coarse siliciclastic sand is common in domes at all depths except on the inner shelf, where ooids are dominant. Quartz and metamorphic rock pebbles and granules are locally present in the thrombolites and stromatolites of the shelf-edge and upper slope, and boulders form nuclei of some of these domes. Carbonate grains, mainly ooids and peloids, are present in all domes. However, the dominant carbonate component is mainly micrite in the form of micro-bushes and clotted fabrics. Important controls in the formation of these siliciclastic stromatolites and thrombolites were: 1) supply of siliciclastic grains, and 2) early carbonate cementation to maintain the relief of the domes during formation and to preserve primary fabrics after burial. Depth-limited microbial components of the mats, probably algae, may have assisted in trapping coarse sediment. They were also possibly responsible for limitation of the thrombolites to the shallow shelf environment.

Chronology and paleohydrology of late Quaternary high lake levels in the Manyara basin (Tanzania) from isotopic data ( 18O, 13C, 14C, [formula omitted]) on fossil stromatolites

Superimposed phases of stromatolite buildup are observed on the eastern margin of Lake Manyara and depict a paleoshoreline at about 20 m above modern lake level. Radiocarbon and Th U measurements permitted the dating of the last two phases of stromatolite formation at ca. 90,000 yr and between 27,000 and 23,000 yr B.P., respectively. The Th U chronology is based on the decay of a strong 230Th-excess (over 234U) inherited with the detrital particles cemented into the stromatolites. The various generations of stromatolites show comparable stable carbon and oxygen isotope contents and are located at the same paleolake stabilization levels. This indicates that stringent hydrological conditions are necessary for the development of the encrusting benthic microbial communities responsible for stromatolite formation. A comparison with similar stromatolitic units from the nearby Lake Natron-Lake Magadi basin shows that such conditions occurred during only a few of the late Quaternary humid episodes known in eastern Africa and that they are different in each basin. Stromatolites do not necessarily represent all high lake levels that Lake Manyara experienced during the late Pleistocene and Holocene.

Laminated cyanobacterial mats in sediments of solar salt works: some sedimentological implications

ABSTRACTFormation of microlaminated sediments in solar salt works along the Mediterranean coast in southern France only occurs within a restricted salinity range of 60–150 gl−1. These salinities are associated with development of a laminated cyanobacterial mat composed primarily of the filamentous cyanobacteria Microcoleus chthonoplastes interbedded with detrital laminae. Transplants of the cyanobacterial mat to a less saline zone (36–60 gl−1) indicated that the cyanobacterial mats failed to colonize the less saline waters due to herbivorous snails and competition for light from floating algal masses of Cladophora and Enteromorpha. Neither the snails nor the Cladophora and Enteromorpha masses are tolerant of salinities above 60 gl−1, and therefore the Microcoleus mats are restricted to those areas of the solar salt works with these higher salinities.Analyses of salinity, conductivity, dissolved oxygen and pH in shallow salt pans (with salinities of 60–150 gl−1) established a relationship between the daily development of oxygen supersaturation and cyanobacterial photosynthesis.Sediments are unlaminated in those portions of the solar salt works where there are no cyanobacterial mats. These mats are frequently drained of their overlying water, and thus desiccation cracks divide them into polygonal plates. The development and translocation of these plates is enhanced by gas bubbles which form under the surface of the mats. No correlation between the microlaminae in sections from two cores located approximately 1 m apart was observed. This was consistent with the hypothesis that the surface of the desiccation crack polygons can be removed by currents and redeposited on the top of other cyanobacterial mat polygons. This process results in a ‘patchwork quilt’of young and old cyanobacterial mat polygons with an irregular microlamination pattern. The presence of such an irregular pattern of laminae permits an important distinction to be made between sediments associated with stromatolite formation and those associated with the very fine and horizontal varved sediments of stratified (meromictic) water bodies. The sedimentological significance of these observations is reviewed in relation to the processes of stromatolite genesis.

Lake Hoare, Antarctica: sedimentation through a thick perennial ice cover.

Lake Hoare in the Dry Valleys of Antarctica is covered with a perennial ice cover more than 3 m thick, yet there is a complex record of sedimentation and of growth of microbial mats on the lake bottom. Rough topography on the ice covering the lake surface traps sand that is transported by the wind. In late summer, vertical conduits form by melting and fracturing, making the ice permeable to both liquid water and gases. Cross-sections of the ice cover show that sand is able to penetrate into and apparently through it by descending through these conduits. This is the primary sedimentation mechanism in the lake. Sediment traps retrieved from the lake bottom indicate that rates of deposition can vary by large amounts over lateral scales as small as 1 m. This conclusion is supported by cores taken in a 3 x 3 grid with a spacing of 1.5 m. Despite the close spacing of the cores, the poor stratigraphic correlation that is observed indicates substantial lateral variability in sedimentation rate. Apparently, sand descends into the lake from discrete, highly localized sources in the ice that may in some cases deposit a large amount of sand into the lake in a very short time. In some locations on the lake bottom, distinctive sand mounds have been formed by this process. They are primary sedimentary structures and appear unique to the perennially ice-covered lacustrine environment. In some locations they are tens of centimetres high and gently rounded with stable slopes; in others they reach approximately 1 m in height and have a conical shape with slopes at angle of repose. A simple formation model suggests that these differences can be explained by local variations in water depth and sedimentation rate. Rapid colonization of fresh sand surfaces by microbial mats composed of cyanobacteria, eukaryotic algae, and heterotrophic bacteria produces a complex intercalation of organic and sandy layers that are a distinctive form of modern stromatolites.

Role of algal eukaryotes in subtidal columnar stromatolite formation

Columnar stromatolites were abundant and widespread in the Proterozoic but are exceedingly rare in modern seas. Consequently, the stromatolites at Hamelin Pool in Shark Bay, Western Australia, have been widely interpreted as unique modern analogs of ancient stromatolites constructed by complex communities of cyanobacteria. However, the Shark Bay columnar stromatolites contain sediment that is unusually coarse for stromatolites both ancient and modern, and the subtidal columnar stromatolites have a significant component of algal eukaryotes dominated by motile diatoms with mucilaginous tubes. This suggests that Shark Bay columnar stromatolites are not strict analogs for most ancient cyanobacterial stromatolites, least of all for those from subtidal environments. We argue that algal eukaryotes may play a substantial role in the formation and maintenance of subtidal columnar stromatolites at Shark Bay and are capable of trapping coarse sediment. In contrast, cyanobacteria have difficulty in trapping coarse sediment and produce essentially fine-grained stromatolites. We propose that there are two major types or end members of Recent marine stromatolites: (i) eualgal-cyanobacterial stromatolites that are generally coarse-grained, and (ii) cyanobacterial stromatolites that are generally fine-grained.

Depositional Environments of Katakturuk Dolomite and Nanook Limestone, Arctic National Wildlife Refuge, Alaska: ABSTRACT

The Katakturuk Dolomite (Proterozoic) and the unconformably overlying Nanook Limestone (Proterozoic. to Devonian) are considered to have the best hydrocarbon reservoir potential of the pre-Mississippian basement complex rocks in the coastal plain subsurface of the Arctic National Wildlife Refuge, Alaska. Where well exposed in the Sadlerochit and Shublik Mountains to the south, these formations display a wide range of basin to shelf carbonate depositional environments. The lower part of the Katakturuk Dolomite consists of debris-flow doliomitic megabreccias and dolomitic to lime mudstone turbidites deposited in a deep-water slope to basin-plain setting. A transition from basin to shelf environments upsection is marked by the presence of ooids (derived from active shelf margin shoals) in turbidites, debris flows, and grain flows. These grade upward into shallow water, crossbedded oolitic and algal grainstones, with intermittent zones of subtidal stromatolites, and culminate in intertidal to supratidal facies containing numerous stromatolite forms, cryptalgal laminate, mudcracks, miscrospeleotherms, and collapse breccias.

Water chemistry of the coastal saline lakes of the Clifton-Preston Lakeland system, south-western Australia, and its influence on stromatolite formation

The water chemistry of Lake Clifton, the adjacent lakes and the regional ground water was investigated to aid in the elucidation of the factors responsible for the restriction of living stromatolites to Lake Clifton. The ionic composition of water in the lakes is proportionally similar to sea water, but the ground water is enriched in calcium and bicarbonate and is of lower salinity (1-2 g I-1). The salinities of the lakes ranged from 7 to 369 g l-1 during 1984 but, in contrast to the other lakes, Clifton remained less saline than sea water throughout the year. Ground waters from an unconfined aquifer on the eastern shore made a large contribution to the annual lake water budget of Lake Clifton, maintaining lake water salinity at less than 35 g l-1 and modifying the chemical composition of the sediment-water interface where stromatolites form. Living, lithified stromatolites occur along the eastern shore of Lake Clifton. They are formed by a benthic microbial community rich in Scytonema. The Stromatolites co-exist with an abundant metazoan fauna, but do not appear to be limited by grazing. Clearly defined zones of ground-water intrusion were found along the eastern foreshore and areas of differential ground-water discharge were associated with morphologically distinctive stromatolites. Occurrence of stromatolites and regions of ground-water discharge in Lake Clifton are consistently associated. It is suggested that the intruding ground waters in Lake Clifton provide a chemical environment conducive to the formation of calcified stromatolites.

Stromatolites and earth—sun—moon dynamics

Inclination of stromatolite columns, caused by nonvertical direction of averaged incident solar radiation, provides a signal for deducing astronomical and geophysical data at time of stromatolite formation. A sample of Anabaria juvensis (Bitter Springs Formation, central Australia) shows a sinusoidal growth pattern which, interpreted as an annual signal (obliquity of the ecliptic) with daily production of laminae, indicates ∼ 435 days per year, a result consistent with other available estimates. A sample of Inzeria from the same formation also shows a sinusoidal pattern and a similar estimate for days per year. Both samples indicate growth rates of centimeters per year. Several laboratory measurement techniques offer systematic procedures for extracting data.

Stromatolites — the challenge of a term in space and time

Abstract Several definitions of stromatolites are discussed and Kalkowsky's most important statements about stromatolites are translated from German to English. A new definition for stromatolites is proposed, namely “Stromatolites are laminated rocks, the origin of which can clearly be related to the activity of microbial communities, which by their morphology, physiology and arrangement in space and time interact with the physical and chemical environment to produce a laminated pattern which is retained in the final rock structure”. Unconsolidated laminated systems, clearly related to the activity of microbial communities and often called “recent stromatolites” or “living stromatolites”, are defined as “potential stromatolites”. The main microbial activities which are important in the formation of potential stromatolites and stromatolites are described and examples of stromatolites of various types, including iron stromatolites, are also described.

Aspects of Proterozoic stromatolite biostratigraphy in Western Australia

Stromatolites are abundant at many horizons in the Proterozoic of Western Australia. Recent advances in knowledge of Proterozoic stratigraphy of the state have provided a more detailed framework for interpreting the stromatolite data than has been available previously. In the 1.7 Ga Earaheedy Group of the Nabberu Basin a characteristic stromatolite assemblage occurs, and within the basin a biostratigraphic succession can be recognized. The assemblage contains several new forms which belong to new groups. The need to erect new groups for these early Proterozoic stromatolites is in agreement with recent studies in Canada, northern Europe and South Africa, and suggests that the problem of ‘younger’ or late Proterozoic stromatolite groups in early Proterozoic rocks mentioned by previous workers is a result of a lack of rigour in defining taxa. Examination of type material is necessary to determine how closely the Earaheedy forms resemble those described from these other regions. In Western Australia some stromatolite forms have a restricted vertical range and similar taxa occur in beds of approximately the same age in widely-separated areas: e.g. Kimberley Group and Earaheedy Group; Scorpion Group and Limbunya, Birrindudu, McArthur, Mt. Rigg and Mt. Albert Groups and Bungle Bungle Dolomite; Tolmer and Bullita Groups; Moora and Bangemall Groups; Kai Ki Beds, Louisa Downs, Mount House and Albert Edward Groups. Stromatolite diversity shows a decline in the number of taxa at about 1.1. Ga in the Bangemall Group. More data are required to determine whether this decline is universal or specific to the Bangemall Group. This study indicates that a stromatolite biostratigraphy for Western Australia is feasible and is consistent with data from other parts of Australia. Thus emphasis on correlation should be placed on the stromatolite form rather than the group, and intercontinental correlations should be attempted only when local biostratigraphic schemes have been firmly established.

Calcification in a coccoid cyanobacterium associated with the formation of desert stromatolites

ABSTRACTIn the Borrego Desert (California) and in the Sinai Desert (Israel) laminated, microbially mediated carbonate crusts have been found and analysed biologically and mineralogically, and further studied with scanning electron microscope methods combined with energy dispersive X‐ray analyses. All morphological and biological features of the extant crusts justify the term ‘desert stromatolite’, a term applied to stromatolites from desert regions which form under permanent exposure to the atmosphere. These stromatolites are never covered by standing water, and running water (heavy rainfall) covers them for only a few hours during the year. Carbonate deposition is achieved principally by the cyanobacterium Pleurocapsa sp. which exhibits characteristic yet different stages of calcification. Calcification occurs in the sheaths of single cells (including baeocytes) as well as in mature colonies. The specificity for calcification in Pleurocapsa sp. is discussed.

Lichen Stromatolites: Criterion for Subaerial Exposure and a Mechanism for the Formation of Laminar Calcretes (Caliche)

ABSTRACT Endolithic and epilithic lichens are pioneer colonizers of exposed calcrete (caliche) hardpans and fresh rock surfaces throughout the coastal regions of the western Mediterranean. Petrographic studies reveal that saxicolous (rock substrate) lichens cause characteristic textural and fabric changes of the colonized substrate. As a result of biophysical disintegration, biochemical decomposition and biosynthesis of mineral components, protosoils are formed. Induration of these superficial, biologically weathered rinds, followed by further lichen colonization, leads to the formation of lichen stromatolites. The effect of saxicolous lichens on exposed, indurated calcrete surfaces has been documented petrographically in terms of textural, fabric and compositional changes with respect to the underlying unaffected part of the calcrete profile. Such documentation provides criteria for the recognition of subaerially exposed substrates and is, thus, of potential importance in helping to delineate former land surfaces in ancient successions. Laminar calcretes contain organic-rich and organic-poor millimetre laminae, microborings, algal filaments, fungal hyphae, calcite spherulites, spherical structures and decimicron-sized calcite grains arranged in surface parallel layers. Hitherto, these features have been reported by numerous workers but their origin has not been considered. Comparative studies with modern lichens suggest that laminar calcretes examined here are lichen stromatolites, that is, that they owe their origin to successive cycles of organic (lichen) telogenesis, incipient pedogenesis and diagenesis. Recognition of lichen stromatolites has far reaching implications in terms of palaeoenvironmental reconstructions. Lichen stromatolites demarcate subaerial discontinuity surfaces, thus indicating terrestrial conditions and duplicating palaeotopography.

Regressive Stromatolite Reefs and Associated Facies, Middle Goulburn Group (Lower Proterozoic), in Kilohigok Basin, N.W.T.: An Example of Environmental Control of Stromatolite Form

ABSTRACT The Goulburn Group is a thick succession of intracratonic sedimentary rocks deposited within the lower Proterozoic Kilohigok Basin, northwestern Canadian Shield, which have been correlated with strata of the Coronation Geosyncline and the Athapuscow aulacogen. The middle part of the Goulburn Group is comprised of mudstone, carbonate and stromatolitic carbonate rocks and is classified as the 'carbonate-mudstone stratigraphic interval' of the Kilohigok Basin. This interval of sedimentation can be subdivided into early and late stages. The late stage consists of a vertical succession of facies representing: basinal mudstone-dominated and carbonate-dominated environments, a shallow submarine shelf with clastic carbonate and thick mudstones, a complex stromatolite reef and associated stromatolite facies and intertidal-supratidal mudstone. These facies represent a diachronous sequence of once laterally juxtaposed environments which is interpreted to represent a major regression that affected the entire Kilohigok Basin. Stromatolite forms characterize different stratigraphic assemblages that correspond with changes in environmental conditions. Reconstruction of the lateral distribution of a now vertical succession of stromatolitic carbonates shows a basinward to landward pattern of: fore-reef clastics with subtidal stromatolite mounds, reef tract with large subtidal mounds and channeled columns, and a quiet-water back-reef with stromatolite biscuits. Stromatolite elongations are interpreted to define paleo-reef-tract trends.